Performance enhancements, readme update

Hey, I made a couple of changes:

1) Added an Express example to the readme
2) Made the file extension check skip to the end of the string instead of checking the entire thing
3) Made your IO entirely asynchronous (although the SSI module still uses synchronous IO for includes :(

I'm also interested in making the SSI module asynchronous, but it might be easier to just switch to https://www.npmjs.org/package/node-ssi - the only feature it appears to be missing is virtual includes, and that should be much easier to add then completely rewriting SSI for async IO. What do you think? Would you accept another PR that switched to node-ssi?